One problem encountered in high-density plasma reactors used to process semiconductor wafers in the production of integrated circuits is electrical charge damage to certain integrated circuit features on the wafer. Charge damage is caused by the difference in velocity distributions of electrons and ions near the wafer surface in surface features having relatively high aspect ratios. The velocity distribution of the plasma ions is vertical, due to the vertical electric field lines near the surface of the wafer induced by an RF bias signal applied to the wafer. The velocity distribution of the plasma electrons approaches an isotropic distribution despite the vertical electric field lines near the wafer surface because of the high electron temperature characteristic of a high density plasma reactor. Thus, in very deep narrow holes, such as contact openings, the vertical trajectory of the ions enables them to travel completely down the entire depth of the opening, so that they nearly all strike the bottom of the opening. In contrast, the nearly isotropic velocity distribution of the electrons enables them to strike the sidewall of the opening, so that only a relatively small portion of the electrons are able to strike the bottom of the opening. As a result, the bottom of the opening acquires a positive charge while the top of the opening acquires a negative charge. As electron temperature increases, this effect is exacerbated and the resulting internal electric fields within the microelectronic features on the wafer can damage those features, resulting in device failure.
RF plasma source power modulation, sometimes referred to as plasma pulsing, is a well-known technique for modifying average plasma electron temperature and plasma chemistry, by pulsing (time-modulating) the RF plasma source power signal. This technique provides some control of electron temperature independent of the RF plasma source power level. This is because the electron temperature decreases at a much quicker rate than plasma density during the power off time between pulses. This control is gained by choosing an appropriate pulse width and pulse repetition rate of the pulse-modulated plasma RF power source so as to reduce the electron temperature without having to reduce the power level.
Pulse-modulating the RF plasma source power has the advantage of not requiring a drastic change in power level to reduce electron temperature. However, this technique does require the RF plasma source power to be modified by pulse modulation. It is a goal of the present invention to reduce plasma electron temperature without having to modify the plasma source power generator (e.g., by the introduction of pulse modulation).
In the present invention, electron temperature is controlled by inducing or enhancing natural instability waves in the plasma.